It has been long known that a bluff (unstreamlined) body disposed within a fluid stream sheds vortices from itself under the principles of the von Karman vortex trail phenomenon. Such vortex shedding is the result of the formation of fluid boundary layers along the outer surface of the body. The lack of streamlining of the body prevents the flow in the boundary layers from following the contour of the body and the flow detaches itself from the body and rolls itself into a series of vortices. The vortices are shed alternately on opposite sides of the body in a periodic manner, the ensuing flow pattern consisting of a spatially oscillating trail of vortices otherwise known as a von Karman vortex trail. It has also been long known that such vortex shedding is related to the magnitude of the flow responsible therefor. In the case of a cylinder placed in a fluid stream, vortex shedding occurs at a frequency proportional to the flow velocity and inversely proportional to the cylinder diameter, the frequency being expressed as: EQU N=S(V/D)
wherein:
N is the frequency at which the vortices detach themselves from one side of the cylinder;
V is the free stream velocity of the fluid flow;
D is the diameter of the cylinder; and
S is the Strouhal Number which is generally constant over a wide range of Reynolds Numbers.
The prior art is replete with fluid flow measuring transducers which seek to provide signals indicative of flow velocity by measuring the frequency at which vortices are shed from a blunt body. Numerous schemes have been proposed for indirectly detecting vortices for the measurement of vortex shedding frequency and hence flow rate. Some of these schemes include detecting vortices by detecting thermal fluctuations of a heated wire in the flow stream, detecting the modulation of an ultrasonic beam in the flow stream, detecting pressure pulses associated with vortex shedding using piezoelectric pressure sensors, the magnetic sensing of the displacement of a metallic shuttle, capacitive detection of the displacement of a flexible membrane and strain guage detection of forces on a tail piece disposed in the flow. For one reason or another, such arrangements for detecting vortices have met with only limited success. For example, flow meters employing sensors sensitive to electromagnetic interference are often unusable in an environment such as within an aircraft or within an industrial environment characterized by high levels of ambient electrical noise.
To overcome some of the shortcomings of the hereinabove mentioned and other prior art vortex shedding flow meters, flow meters which optically detect the presence of vortices in the flow have been introduced. Such optical vortex shedding flow meters have sought to detect the presence of vortices as, for example, by detecting the modulation by vortices of a light beam through the flow, detecting the modulation of the refractive index of the fluid resulting from the presence of vortices therein and detecting the modulation of a light beam by a body which vibrates as a result of vortices acting thereon. These optical flow meters have also met with only limited acceptance. Various of those optical flow meters which rely on the vortex itself to modulate a light beam may, under certain circumstances, not be sufficiently accurate. Those flow meters which rely on a vibratory body to modulate a light beam may be insufficiently reliable and maintainable due to, for example, rapid wear of guides, stops or other means for limiting the movement of the body.
Generally, any flow meter which indirectly detects the presence of vortices, either optically or otherwise, is subject to noise in the fluid stream which adversely effects the accuracy and sensitivity of the meter.